Thermal processing system with supplemental resistive heater and shielded optical pyrometry

ABSTRACT

System and method for determining thermal characteristics, such as temperature, temperature uniformity and emissivity, during thermal processing using shielded pyrometry. The surface of a semiconductor substrate is shielded to prevent interference from extrinsic light from radiant heating sources and to form an effective black-body cavity. An optical sensor is positioned to sense emitted light in the cavity for pyrometry. The effective emissivity of the cavity approaches unity independent of the semiconductor substrate material which simplifies temperature calculation. The shield may be used to prevent undesired backside deposition. Multiple sensors may be used to detect temperature differences across the substrate and in response heaters may be adjusted to enhance temperature uniformity.

RELATED APPLICATION

This application is a continuation of application Ser. No. 08/451,789,filed May 26, 1995, now U.S. Pat. No. 5,830,277.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The field of the present invention relates in general to semiconductorprocessing. More particularly, the field of the invention relates to asystem and method for detecting and measuring semiconductor substrateproperties, such as temperature, temperature uniformity, and emissivity,using optical pyrometry.

2. Background

In rapid thermal processing (RTP) of semiconductor device materials(such as a semiconductor wafer or other substrate), one of the criticalprocess parameters is temperature. Repeatable, precise, andprocess-independent measurements of the wafer temperature are among themost important requirements of semiconductor processing equipment (suchas RTP) in integrated circuit manufacturing.

Contact temperature sensors, such as thermocouples and thermistors, arecommonly used to measure temperature. However, these sensors are notwell-suited to many wafer processing environments. Such temperaturesensors typically must be placed in contact with a wafer which mayaffect the uniformity of heating and expose the wafer to contaminantsunder certain conditions. In addition, it is difficult to achieverepeatable temperature measurement conditions due to inconsistentcontact areas and other variations in heat transfer to the sensor.

As a result, noninvasive temperature measurement techniques, such asoptical pyrometry, have been used in many RTP systems. Unliketemperature measurement using contact sensors, such as thermocouples andthermistors, temperature measurement using optical pyrometry does notrequire contact with the wafer and as a result does not expose the waferto metallic contaminants during processing. Optical pyrometers maydetermine temperature based upon optical electromagnetic radiation(hereinafter “light”) emitted from an object. Optical pyrometerstypically use a high temperature optical fiber, light pipe, lens, orother light collecting device to transmit light to a light sensitivedevice that measures the flux density or intensity of the light emittedby the object. See, e.g., U.S. Pat. No. 4,859,079 to Wickerstein et al.and U.S. Pat. Nos. 4,750,139 and 4,845,647 to Dils et al., each of whichis incorporated herein by reference. The temperature is then determinedusing Planck's equation which defines the relationship between thetemperature of an object, the flux density of light being emitted fromthat object, and the light emitting characteristics of the object'ssurface (emissivity).

The advantages of optical pyrometry for RTP include its noninvasivenature and relatively fast measurement speed which is critical incontrolling the rapid heating and cooling in RTP. However, accurateoptical temperature measurement using pyrometry depends upon theaccurate measurement of the flux density of light emitted from the waferand upon the wafer's light emitting characteristics or emissivity.Emissivity is typically wafer dependent and depends on a range ofparameters, including temperature, chamber reflectivity, the wafermaterial (including dopant concentration), surface roughness, andsurface layers (including the type and thickness of sub-layers), andwill change dynamically during processing as layers grow on the surfaceof the wafer.

Among other things, the emissivity of a semiconductor wafer depends uponthe wavelength of light that is being measured. FIG. 1 is a graph ofemissivity as a function of wavelength and temperature for a puresilicon wafer. As shown in FIG. 1, emissivity is temperature dependentand may vary greatly at wavelengths greater than the absorption band ofsilicon, which is slightly less than one and two tenths (1.2)micrometers. The effects of emissivity with respect to temperature onoptical pyrometry for silicon can be minimized by using a sensor withmaximum sensitivity within a range of about eight tenths (0.8) to aboutone and one tenth (1.1) micrometers wavelength, as indicated at 100 inFIG. 1. See also U.S. Pat. No. 5,166,080 to Schietinger et al. which isincorporated herein by reference. However, the wavelength/emissivitycharacteristic of a wafer will differ for doped silicon and othersemiconductor materials such as gallium arsenide. In addition, theemissivity of a given wafer will typically change during processing asmaterials, such as silicon dioxide, are deposited on the wafer surface.Therefore, it is very difficult to control the effects of emissivity ontemperature measurement based solely upon the wavelength of light thatis used for pyrometry.

A short wavelength less than one (1) micrometer is often preferred foroptical pyrometry since it provides certain benefits. For instance, ashort wavelength improves the sensitivity of the temperature measurementwhich is based on Planck black-body emission. Sensitivity is defined asthe fractional change in radiance per fractional change in temperatureand from the equations for Planck black-body emission it can be shownthat sensitivity is inversely proportional to wavelength. Therefore,shorter wavelengths are preferred for improved sensitivity intemperature measurement.

However, the ability to use a short wavelength of light in opticalpyrometry is severely limited in many conventional rapid thermalprocessors due to interference from radiant energy heating sources. Theheat needed for RTP is typically provided by a heating lamp module whichconsists of high intensity lamps (usually tungsten-halogen lamps or arclamps). FIG. 2 illustrates a conventional RTP processing chamber,generally indicated at 200, using two banks of heating lamps 202 and 203to heat a semiconductor wafer 204 through optical windows 206 and 207.An optical pyrometer 208 may be used to measure the wafer temperature bydetecting the flux density of light within cone of vision 210. However,most radiant energy heating sources, including tungsten filament and arclamp systems, provide their peak energy intensity at a wavelength ofabout one micrometer which interferes with optical pyrometer 208.Optical pyrometer 208 will detect light reflected off of wafer 204 fromheating lamps 202 and 203 as well as light emitted from the wafer. Thisreflected lamp light erroneously augments the measured intensity oflight emitted from the wafer surface and results in inaccuratetemperature measurement.

Therefore, some conventional systems have used a longer wavelength oflight to measure temperature so that the spectral distribution of theheating lamps has minimal overlap with the pyrometer's operatingspectral band or wavelength. However, measuring the intensity of emittedlight at a longer wavelength to reduce interference can lead to similarinterference problems from light emitted by hot chamber surfaces which,at longer wavelengths, becomes significant enough to cause errors in themeasured light intensity. For instance, quartz, which is typically usedin RTP processing chambers, re-emits light predominantly at longerwavelengths (typically greater than 3.5 micrometers). An alternativeapproach is to measure and compensate for reflected light. Two opticalpyrometers may be used—one for measuring the light from the lamps andone for measuring the light from the wafer. The strength of thecharacteristic AC ripple in light emanated from the lamp can be comparedto the strength of the AC ripple reflected from the wafer to determinethe wafer's reflectivity. This, in turn, can be used to essentiallysubtract out reflected light in order to isolate the emitted light fromthe wafer for determining temperature using Planck's equation. See,e.g., U.S. Pat. No. 5,166,080 to Schietinger et al. However, suchsystems may require complex circuitry to isolate the AC ripple andperform the calculations that effectively eliminate reflected light.Such systems also require an additional light pipe and other components.

FIG. 3A illustrates an RTP processing chamber, generally indicated at300, with a single set of heating lamps 302 for heating wafer 304.Typically, wafer 304 is supported by low thermal mass pins (not shown).An optical pyrometer 308 may be placed behind the wafer opposite theheating lamps 302. This reduces, but does not eliminate, theinterference from lamps 302. In addition, wafer 304 may remain at leastpartially transparent to lamp radiation in the infrared region (beyond1.5 micrometers) at lower wafer temperatures (below 600° C.) sopyrometer 308 may still be affected by lamp radiation or radiationre-emitted from quartz that is transmitted through wafer 304. FIG. 4 isa graph of silicon wafer transmission as a function of wavelength at atemperature of twenty five degrees Celsius (25° C.). As can be seen,silicon transmission is greatly reduced for wavelengths less than aboutone and two hundredths (1.02) micrometers. Therefore, transmissionproblems can be substantially eliminated by using short wavelengths(which also provide advantages for emissivity and sensitivity asdescribed above), but interference from heating lamps 302 is alsogreatest at such wavelengths.

FIG. 3B illustrates an alternative single-sided lamp RTP processingchamber, generally indicated at 350. This processing chamber 350includes a backside shield 352 for preventing deposition on the backsideof wafer 354. This is desirable in many processing applicationsincluding chemical vapor deposition (CVD). In contrast, systems that useonly low thermal mass pins to support the wafer allow material to bedeposited everywhere on the wafer, including the backside which isexposed. Preventing such backside deposition usually simplifies theoverall semiconductor device fabrication process. As shown in FIG. 3B,such systems may use an optical pyrometer 356 aimed at the backsideshield to measure the approximate wafer temperature. While the shieldeliminates wafer transmission problems, it also reduces the accuracy oftemperature measurement since it is the temperature of the shield, andnot the wafer, that is actually being measured.

For both double-sided and single-sided lamp RTP systems, additionalproblems may be introduced if the optical pyrometer samples emittedlight through a window or lens that is exposed to the processingchamber. During certain processes, deposits may form on the window orlens which inhibit the transmission of light to the optical pyrometer.The optical pyrometer will sense less light due to the deposits andproduce a temperature measurement that is lower than the actualtemperature of the wafer. This may cause the lamp control system toerroneously increase the heat provided to the chamber to compensate forthe falsely detected low temperature measurement. As the heat isincreased, deposits may accelerate leading to thermal runawayconditions.

Another approach to compensating for the effects of emissivity ontemperature measurement is to use a reflective cavity. A processingchamber may be designed to reflect emitted light from remote, highlyreflective walls. The emitted light is radiated diffusely and reflectedover the entire wafer numerous times. The effective emissivity of thewafer in such a system is determined by integrating the reflected lightintensity over the wafer surface. By using a high aspect ratio for thereflective cavity, the effective emissivity of a wafer is increasedtoward unity which helps eliminate the effects of edges, device patternson the wafer, and backside roughness. However, such an approachrestricts chamber geometry and may not be practical in cold wall RTPchambers. The reflective cavity approach typically treats the wafer asan extension of the chamber wall. However, a wafer should not makecontact with, or be too closely spaced to, a cold wall in RTP systems toavoid uneven cooling at the wafer edges. Further, reflective walls mayinterfere with rapid cooling in RTP systems which may require walls thatrapidly absorb energy rather than reflect it. In addition, deposits mayform on reflective walls during some processes which may diminishreflectivity and thereby introduce error into the temperaturemeasurement.

Another approach is to place a small black-body enclosure around the tipof an optical sensor. The black-body enclosure heats up to theapproximate temperature of the surrounding environment and emits lightinto the optical sensor that is proportional to the temperature of theblack-body enclosure. However, such an approach measures the temperatureof the black-body emitter and not the wafer. As with thermocouples, suchan approach typically requires that the temperature sensor be placed incontact with the wafer, and repeatable accurate temperature measurementsare difficult to achieve under actual processing conditions.

Yet another approach for measuring wafer temperature and compensatingfor emissivity uses an infrared laser source that directs light into abeam splitter. From the beam splitter, the coherent light beam is splitinto numerous incident beams which travel to the wafer surface viaoptical fiber bundles. The optical fiber bundles also collect thereflected coherent light beams as well as radiated energy from thewafer. In low temperature applications, transmitted energy may becollected and measured as well. The collected light is then divided intoseparate components from which radiance, emissivity, and temperature maybe calculated. See, e.g., U.S. Pat. No. 5,156,461 to Moslehi et al. Itis a disadvantage of such systems that a laser and other complexcomponents are required. Such systems, however, are advantageous becausethey may provide measurements of wafer temperature at multiple pointsalong the wafer surface which may be useful for detecting andcompensating for temperature nonuniformities.

Advanced fabrication processes demand uniform temperature across thewafer with gradients preferably less than plus or minus two degreesCelsius (±2° C.) to provide for uniform processing and to avoid thermalinduced stress which may cause crystal slip in the wafer. However, RTPtypically requires low thermal mass to allow for rapid heating andcooling. Such systems use a cold-wall furnace with radiant energysources to selectively heat the wafer. While this allows rapid heatingand cooling, the temperature uniformity becomes sensitive to theuniformity of the optical energy absorption as well as the radiative andconvective heat losses of the wafer. Wafer temperature non-uniformitiesusually appear near wafer edges because radiative heat losses aregreatest at the edges. During RTP the wafer edges may, at times, beseveral degrees (or even tens of degrees) cooler than the center of thewafer. The temperature nonuniformity may produce crystal slip lines onthe wafer (particularly near the edge). Slip lines are collections ofdislocations in the crystal lattice structure of the silicon caused byunequal movement of atomic planes due to thermally induced stress. Thismay result in the formation of electrically active defects which degradethe circuitry and decrease yield. Therefore, a system for detecting andcorrecting temperature differences across the wafer is often required.In particular, accurate, multi-point optical pyrometry is desirable fordetecting temperature differences which can then be corrected.

Correction or compensation for temperature nonuniformities has beenprovided using several techniques. One approach is to use a multi-zonelamp system arranged in a plurality of concentric circles. The lampenergy may be adjusted to compensate for temperature differencesdetected using multi-point optical pyrometry. However, such systemsoften require complex and expensive lamp systems with separatetemperature controls for each zone of lamps. Another approach has beento place a ring of material (such as silicon or the like) around, and incontact with, the periphery of the wafer. The ring provides extrathermal insulation to retain heat at the periphery of the wafer, buttypically does not offer sufficient flexibility over a wide range oftemperatures.

What is needed is an RTP processing chamber with an accurate opticalpyrometry system for measuring semiconductor substrate temperature.Preferably such a system would provide optical pyrometry at shortwavelengths without interference from direct lamp light or reflected orre-emitted light from chamber surfaces. Further, such a system wouldpreferably be emissivity insensitive to allow accurate temperaturemeasurement in a variety of processes using different semiconductormaterials, dopants, and layers and would not require complex emissivityand reflectivity measurement and compensation systems as do manyconventional approaches. In addition, such a system would preferablyallow an optical pyrometer to measure the intensity of emitted lightwithout interference from coatings or other obstructions on windowswhich may lead to thermal runaway conditions.

What is also needed is an RTP processing system and method forpreventing undesired backside deposition. Preferably, such a systemwould prevent backside deposition on a semiconductor substrate whilestill allowing direct multi-point temperature measurement. What is alsoneeded is a system and method for detecting and correcting temperaturedifferences across a semiconductor substrate due to edge losses withoutrequiring complex multi-zone lamp systems. Preferably, each of thesefeatures would be combined in a single, cost-effective RTP processingsystem and method.

SUMMARY OF THE INVENTION

A first aspect of the present invention provides a semiconductorsubstrate processing system and method using shielded optical pyrometryto prevent interference from extrinsic light. Preferably, a shield ispositioned relative to a semiconductor substrate such that a shieldedregion is formed between the surfaces of the substrate and the shield.The shield and the substrate substantially prevent reflected lightwithin a given range of wavelengths from entering the region andinterfering with the optical pyrometry. The intensity of light withinthe given range of wavelengths emitted by the semiconductor substrateinto the region may then be sampled to measure substrate temperature andto detect temperature differences across the substrate surface. It is anadvantage of these and other aspects of the present invention thatinterference with optical pyrometry due to reflected light within theprocessing chamber or coatings on the windows and walls is substantiallyeliminated.

Another aspect of the present invention provide a shield and asemiconductor substrate that are substantially nontransmissive to lightwithin the given range of wavelengths at processing temperatures. It isan advantage of this aspect of the present invention that interferencewith optical pyrometry due to light transmitted through the substrate orshield into the shielded region is substantially eliminated.

It is a further advantage of the above aspects of the present inventionthat the shielded region may be formed to approximate an ideal cavityradiator under many processing conditions with an effective emissivityapproaching unity. Thus, temperature may be accurately measuredsubstantially independently of the intrinsic emissivity of thesemiconductor substrate which may vary across materials andtemperatures. It is a further advantage that short wavelength radiantenergy heating sources may be used in conjunction with short wavelengthoptical pyrometry without interference from reflected and transmittedlight.

Another aspect of the present invention provides a shield thatsubstantially prevents reactive gas (or other substances) from enteringthe shielded region and forming deposits on an optical sensor, or awindow for an optical sensor, which is used to sample the intensity ofemitted light within the shielded region. It is an advantage of thisaspect of the present invention that the intensity of light may besampled without interference from deposits on an optical sensor orwindow for an optical pyrometer.

Yet another aspect of the present invention provides a shield thatsubstantially prevents reactive gas (or other substances) from enteringthe shielded region and forming deposits on semiconductor substratesurfaces within the shielded region. It is an advantage of this aspectof the present invention that undesired deposition on the backside ofthe semiconductor substrate may be substantially prevented. It is afurther advantage that dynamic changes in the emissivity of thesemiconductor substrate due to the deposition of backside layers areavoided.

Another aspect of the present invention provides a heater and/or thermalinsulation to compensate for temperature differences and uneven heatlosses across the substrate surface. In particular, the heat provided byheaters, such as resistive heaters and lamp modules, may be adjusted toprovide uniform heating in response to the intensity of light sampled atvarious points across the substrate surface. In particular, emittedlight from the substrate may be sampled at various locations within ashielded region as described above. It is an advantage of these andother aspects of the present invention that a more uniform substratetemperature may be achieved.

Yet another aspect of the present invention provides a shield havingrotating and non-rotating portions. The rotating portion may be used torotate the semiconductor substrate during processing and thenon-rotating portion may be used to support an optical sensor thatsamples emitted light within the shielded region. It is an advantage ofthis aspect of the present invention that a shielded region may be usedto block extrinsic light from interfering with optical pyrometry, whilestill allowing the substrate to rotate during processing to provide moreuniform heating. Rotation of the semiconductor substrate is desiredbecause it reduces the impact of localized heating irregularities on thesubstrate by spreading the effect of such irregularities across acircumference of the substrate surface. Rotation also enhances theuniformity of layers deposited on the substrate by evening out localizedirregularities which may be caused by, among other things, uneven gasflow distribution across the substrate surface.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention willbecome more apparent to those skilled in the art from the followingdetailed description in conjunction with the appended drawings in which:

FIG. 1 is a prior art graph with a vertical axis for emissivity, ahorizontal axis for wavelength, and a series of plotted curvesillustrating the emissivity of silicon at a given temperature forvarious wavelengths of light;

FIG. 2 is a side cross sectional view of a prior art rapid thermalprocessing chamber having heating lamps on both sides of a wafer;

FIG. 3A is a side cross sectional view of a prior art rapid thermalprocessing chamber having heating lamps on the top side of a wafer;

FIG. 3B is a side cross sectional view of a prior art rapid thermalprocessing chamber having heating lamps on one side of a wafer and ashield preventing deposition on the other side of the wafer;

FIG. 4 is a prior art graph with a vertical axis for transmission, ahorizontal axis for wavelength, and a plotted curve illustrating thetransmission of a silicon wafer at twenty five degrees Celsius (25° C.)for various wavelengths of light;

FIG. 5 is a partial cross sectional view and a partial block diagram ofa rapid thermal processing system according to a first embodiment of thepresent invention;

FIG. 6A is a side cross sectional view of a rapid thermal processingchamber according to the first embodiment of the present invention;

FIG. 6B is a bottom cross sectional view of a rapid thermal processingchamber according to the first embodiment;

FIG. 7A is a side cross sectional view of a center piece for a radiationshield according to the first embodiment;

FIG. 7B is a bottom plan view of a center piece for a radiation shieldaccording to the first embodiment;

FIG. 7C is a side cross sectional view of a channel piece for aradiation shield according to the first embodiment;

FIG. 7D is a bottom plan view of a channel piece for a radiation shieldaccording to the first embodiment;

FIG. 7E is a side cross sectional view of an outer piece for a radiationshield according to the first embodiment;

FIG. 7F is a bottom plan view of an outer piece for a radiation shieldaccording to the first embodiment;

FIG. 8 is a side cross sectional view of a rapid thermal processingchamber having a plurality of optical sensors according to analternative embodiment of the present invention;

FIG. 9 is a side cross sectional view of a rapid thermal processingchamber with additional thermal insulation at the edge of a waferaccording to an alternative embodiment of the present invention; and

FIG. 10 is a side cross sectional view of a central support and drivemechanism according to the first embodiment of the present invention.

DETAILED DESCRIPTION

One aspect of the present invention allows the temperature, temperatureuniformity, and emissivity of a semiconductor substrate to be accuratelydetected and measured using optical pyrometry. The following descriptionis presented to enable any person skilled in the art to make and use theinvention. Descriptions of specific designs are provided only asexamples. Various modifications to the preferred embodiment will bereadily apparent to those skilled in the art, and the generic principlesdefined herein may be applied to other embodiments and applicationswithout departing from the spirit and scope of the invention. Thus, thepresent invention is not intended to be limited to the embodiment shown,but is to be accorded the widest scope consistent with the principlesand features disclosed herein.

Referring to FIG. 5, in the first embodiment, a semiconductor wafer 502or other semiconductor substrate is placed in an RTP processing chamber504 for processing. Processing chamber 504 is enclosed by walls 506. Thewalls 506 form several openings into the chamber including an inlet 508for reactive gas (or other substances for forming deposits or otherwiseprocessing the wafer), an outlet 510 for a gas exhaust system, a passage512 forming part of a load lock mechanism for loading and unloadingwafer 502, and a hole 514 through which a wafer support mechanism mayenter chamber 504. Lamps 516 or other radiant energy heating sourcesradiate through window 520 to heat wafer 502. A radiation shield 522 isplaced adjacent to the wafer to form a cavity 524. During processing,radiation shield 522 and wafer 502 substantially block all direct andreflected light at the wavelengths used by the optical pyrometer fromentering cavity 524. The wafer and radiation shield are preferablyselected from a material that has low transmission at the wavelengthsused by the optical pyrometer for the desired processing temperatures.Therefore, the light in the cavity at the wavelengths used by theoptical pyrometer is substantially limited to light emitted from thesemiconductor wafer and radiation shield. An optical sensor 526, such asa light pipe or optical fiber, may be introduced into cavity 524 througha small aperture to sample the emitted light. In particular, in thefirst embodiment light emitted at low wavelengths less than three andone half (3.5) micrometers (and preferably less than one and two tenths(1.2) micrometers) may be collected at one or more points of the wafersurface within the cavity and transmitted to an optical pyrometer.Direct interference from the lamps at these wavelengths is blocked bythe radiation shield and wafer.

The cavity formed by the radiation shield and wafer approximates anideal black-body cavity radiator. An ideal cavity radiator consists of acavity within a body where the walls of the cavity are held at a uniformtemperature. Thermal radiation within an ideal cavity, called cavityradiation, has a simple spectrum whose nature is determined only by thetemperature of the walls and not in any way by the material of thecavity, its shape, or its size. That is, the effective emissivity withinan ideal cavity radiator is always equal to one (1).

A black-body cavity is approximated in the first embodiment by placingradiation shield 522 adjacent to wafer 502 which causes the effectiveemissivity within cavity 524 to approach unity substantiallyindependently of the materials used. Thus, different semiconductormaterials with different emissivity values or deposited layers may beused within the first embodiment without substantially affecting theaccuracy of temperature measurements. In addition, conventional problemsassociated with backside roughness and other coatings are substantiallyeliminated.

The radiation shield may also be used to prevent undesired backsidedeposition on a wafer surface and to prevent the emissivity of the waferfrom changing due to the growth of layers on the backside of the wafer,which may be important to the extent that emissivity remains a factor ina system using aspects of the present invention. In addition, the numberof layers or the thickness of the radiation shield may be varied alongthe radius of the wafer to provide varied thermal insulation tocompensate for differences in wafer temperature due to heat losses atthe periphery of the wafer. A peripheral heater 528 (such as a resistivering heater), lamp modules, or other heater arranged about the peripheryof the wafer may also be used to adjust for temperature differences inthe first embodiment.

The system of the first embodiment also includes a variety of subsystemsfor controlling rapid thermal processing including a central support anddrive mechanism 530, a purge gas system 532, an optical pyrometer 534, acomputer system 536, and a heater control 538. The central support anddrive mechanism 530 provides a support system for radiation shield 522and optical sensor 526. The central support and drive mechanism 530 alsoprovides a rotational drive for rotating the wafer during processing, aswell as an elevational mechanism for raising and lowering the wafer forloading and unloading through passage 512. A purge gas system 532introduces purge gas into the central support and drive mechanism 530 toprevent deposits from forming within the mechanism. The purge gas system532 may use conventional gas dispersing techniques.

Optical sensor 526 samples the intensity of light within cavity 524 andprovides an indication of the intensity to optical pyrometer 534. Thissampling may be accomplished by transmitting light from the cavity tooptical pyrometer 534 or by providing a signal to optical pyrometer.534which corresponds to the sampled intensity. Optical pyrometer 534 iscoupled to a computer system 536 for calculating temperature andcontrolling the RTP process. The optical pyrometer 534 and computersystem 536 may calculate wafer temperature and, in certain embodiments,detect temperature differences across the wafer surface. Computer system536 may then send a signal to heater control 538 to adjust the wafertemperature or correct for temperature differences. Heater control 538then regulates the power to lamps 516 and peripheral heater 518 asindicated by computer system 536. It will be readily apparent to thoseskilled in the art that many conventional techniques may be used inoptical pyrometer 534, computer system 536, and heater control 538;although the more stable effective emissivity of cavity 524 may be usedto determine temperature rather than the wafer's intrinsic emissivitywhich may change dynamically during processing.

Although cavity 524 provides a relatively stable effective emissivityfor temperature measurement, the system of the first embodiment may beequipped with a calibration light source 540 which may be used tomeasure the intrinsic emissivity of wafer 502. Although the effectiveemissivity of cavity 524 is substantially independent of the intrinsicemissivity of wafer 502 at thermal equilibrium, the intrinsic emissivitymay be used to provide minor adjustments to enhance accuracy. Suchadjustments may be desirable during temperature ramp up and ramp downwhen cavity 524 may not be at thermal equilibrium and as a result maydeviate from ideal cavity radiator conditions. In addition, ameasurement of a wafer's intrinsic emissivity may be stored and used byother processing chambers that subsequently process the wafer. Themeasured intrinsic emissivity should remain accurate after processing bythe reactor of the first embodiment, since the radiation shield preventsbackside layers from forming which might otherwise alter the intrinsicemissivity.

The intrinsic emissivity may be calculated by projecting light of apredetermined intensity from the calibration light source 540 through anoptical fiber onto the backside of wafer 502 prior to rapid thermalprocessing. Reflected light may be collected by the optical fiber andmeasured by optical pyrometer 534 to determine the reflectivity and inturn the intrinsic emissivity of the wafer.

Aspects of the first embodiment provide a variety of advantages overconventional RTP processing and temperature measurement techniques. Inthe first embodiment, short wavelength optical pyrometry may be usedwithout interference from radiant energy heating sources. Usingwavelengths provides improved sensitivity in temperature measurement andalso reduces interference from light transmitted through the wafer atlow temperatures. In addition, the first embodiment allows temperatureto be accurately measured without complex calculation of dynamicintrinsic wafer emissivity. Rather, aspects of the first embodimentsubstantially eliminate unpredictable dependencies upon a wafer'sintrinsic emissivity which may vary across materials and temperatureranges and which may change as surface layers are deposited duringprocessing. It is a further advantage of the first embodiment thatundesired backside deposition from substances in the processing chamber(which may be provided for front side chemical or physical vapordeposition or sputtering) may be prevented. Multi-point pyrometry andadjustments for temperature nonuniformities may also be included in asystem according to the present invention as described further below.

The structure and operation of an RTP system according to the firstembodiment of the present invention will now be described in furtherdetail. FIG. 6A shows a side-cross sectional view of a rapid thermalprocessing reactor according to the first embodiment and FIG. 6B shows abottom cross sectional view of a rapid thermal processing reactoraccording to the first embodiment. The reactor may be a cold-walledreactor with reactor walls 506 cooled by water cooling channels 607 orsome other cooling system. A semiconductor wafer 502 is placed withinthe processing chamber 504 for rapid thermal processing. In the firstembodiment, wafer 502 is a 200 millimeter silicon wafer. Wafer 502 mayhave materials deposited on it or be doped or coated with any variety oflayers, including silicon dioxide, silicon nitride or the like. It willbe readily apparent to those of ordinary skill in the art that aspectsof the present invention may be applied to substrates having a varietyof sizes and comprising a wide range of materials (including withoutlimitation gallium arsenide, polycrystalline silicon, silicon carbide orthe like). In fact, it is a major advantage that aspects of the presentinvention may be applied across a wide range of materials havingdifferent intrinsic emissivities.

As in many conventional systems, wafer 502 may be rapidly heated byradiant energy heating sources, such as lamps 516, positioned above thewafer 502 and outside of the processing chamber 504. Of course, in otherembodiments radiant energy heating sources or other heaters could beplaced above or below the wafer either inside or outside of theprocessing chamber. Light is radiated from lamps 516 through a quartzwindow 520 onto the top surface of wafer 502 thereby heating the wafer.In the first embodiment, the lamps 516 are conventional tungstenfilament lamps or arc lamps which radiate predominantly at lowwavelengths of approximately one (1) micrometer. Of course otherheaters, such as longer wavelength sources or resistive heaters, may beused in conjunction with the first embodiment. Radiation from lamps 516is reflected within the processing chamber 504 and is also absorbed bychamber surfaces including chamber walls 506, window 520 and wafer 502as well as quartz liners (not shown) which may be used to protect andinsulate the chamber walls. Light may be re-emitted from one or more ofthese surfaces at a variety of wavelengths, including longer wavelengths(greater than one and one half (1.5) micrometers). For instance, whenheated to relatively low temperatures quartz window 520 re-emits lightpredominantly at wavelengths greater than three and one half (3.5)micrometers. Although the processing chamber may contain emitted andreflected light across a wide spectrum of wavelengths, aspects of thefirst embodiment allow the temperature of wafer 502 to be accuratelymeasured using optical pyrometry.

In the first embodiment, a radiation shield 522 encloses the backside ofwafer 502 forming a region such as cavity 524 that is shielded fromlight radiated from lamps 516 and light reflected off of chambersurfaces. As will be described in detail below, this region may be usedto approximate an ideal black-body cavity radiator. An optical sensor526 may be introduced into the shielded region to sample the intensityof emitted light in order to determine the temperature of wafer 502using techniques of optical pyrometry.

In the first embodiment, the center piece 622 a of the radiation shield522 forms a small aperture that allows the optical sensor 526 to samplethe intensity of light within cavity 524. The optical sensor mayactually enter the cavity through the aperture or may be positionedoutside the cavity and sample light that passes through the aperture. Inthe latter case, the aperature need not provide an opening and maysimply be a small window transmissive to light of the wavelength used bythe optical pyrometer. However, an aperture allowing the optical sensorto be directly exposed to light emitted from the wafer—without a windowbetween the optical sensor and the wafer—is preferred to avoidinterference from any deposits on the window or light emitted from thewindow. In embodiments where a window is used to sample the intensity oflight within the cavity, the window is preferably-shielded by theradiation shield or purged to prevent deposits from forming on thewindow.

In the first embodiment, the optical sensor 526 is preferably a sapphirelight pipe in a coaxial purged sheath of silicon carbide 615 although ahigh temperature optical fiber may be used instead. Although a lightpipe or optical fiber is used in the first embodiment, any variety oflight collection, transmission and detection systems—including lens andmirror systems, photodetectors or the like—may be used in conjunctionwith aspects of the present invention. What is desired is an opticalsensor that samples the intensity of light emitted within a shieldedregion. This sampling may be accomplished by collecting and transmittinglight from the shielded region or by providing a signal corresponding tothe sampled intensity. As will be readily apparent to those of ordinaryskill in the art, the term “light” as used herein refers toelectromagnetic radiation within the optical range and is not limited tovisible light.

In the first embodiment, radiation shield 522 preferably comprises amaterial that will substantially block light from entering the cavity524, particularly at wavelengths that are used by the optical pyrometerto determine temperature (which for the first embodiment are wavelengthsless than three and one half (3.5) micrometers and are preferably in therange of eight tenths (0.8) to one and two tenths (1.2) micrometers). Inaddition, it is desirable to use a material for radiation shield 522that has thermal properties similar to wafer 502. In particular, it isdesirable to use a material capable of reaching substantial thermalequilibrium with wafer 502 within a short period of time relative to theperiod of time used for the processing steps. In particular, siliconcomposite materials may provide advantages when used for the radiationshield. In the first embodiment, the radiation shield comprises siliconcarbide, although other materials such as graphite, silicon carbidecoated graphite, silicon, and polycrystalline silicon may also be used.Silicon carbide has advantageous thermal properties and will notcontaminate wafer 502 under most conditions. Silicon carbide has anintrinsic emissivity of approximately nine tenths (0.9) at mostprocessing temperatures.

In the first embodiment, the radiation shield 522 also has a thicknessover most of its area that is within a factor of three (3) of thethickness of wafer 502. In particular, radiation shield 522 has athickness of approximately fifty thousandths (0.050) of an inch overmost of its area in the first embodiment. Since radiation shield 522 hasthermal properties similar to wafer 502 and a thickness on the order ofthe wafer thickness in the first embodiment, the radiation shield 522heats up and attains thermal equilibrium within a time periodsubstantially equal to the time required to heat wafer 502 to a desiredprocessing temperature. Preferably, the temperature ramp up and rampdown time for radiation shield 522 should not lag the temperature rampup and ramp down for wafer 502 by more than a short period of time thatis substantially less than the period of time desired for processingwafer 502 at a given processing temperature (i.e. less than one tenth(0.1) the period of time for the processing step).

In the first embodiment, radiation shield 522 is assembled from fivesubstantially annular or cylindrical pieces of silicon carbide, labeled622 a, 622 b, 622 c, 622 d and 622 e in FIGS. 6A and 6B, although itwill be readily apparent to those of ordinary skill in the art that awide range of configurations may be used for the radiation shield. Whatis desired in the first embodiment is a radiation shield thatsubstantially prevents reflected light within the processing chamberfrom interfering with optical sensor 526. In the first embodiment, acenter piece 622 a is disposed radially about the optical sensor 526,and is supported by a quartz center support 619. The center piece 66 a,center support 619, and optical sensor 526 are stationary in the firstembodiment during processing; however, center piece 66 a, center support619, and optical sensor 526 may be raised and lowered as a unit to raiseand lower wafer 502 for loading and unloading. Center piece 66 a, centersupport 619, and optical sensor 526 may be raised approximately one half(0.5) inch above the outer portion 622 e of the radiation shield. Wafer502 may be loaded into the processing chamber 504 through a conventionalload lock mechanism (not shown) through passage 512 and placed on theraised center piece 622 a. The center piece 66 a is then lowered whichcauses the wafer 502 to be placed as shown in FIG. 6A with edges restingon the outer piece 622 e of the radiation shield. The center piece 66 ais then lowered approximately six hundred and fifty thousandths (0.650)of an inch below the wafer 502 into the position shown in FIG. 6A forprocessing.

FIG. 7A is a side cross sectional view, and FIG. 7B is a bottom planview, of center piece 622 a of the radiation shield. Referring to FIGS.7A and 7B, the center piece 622 a has a cylindrical sheath portion 702that surrounds optical sensor 526 and rests on center support 619. Thecylindrical sheath portion 702 extends approximately three fourths(0.75) of an inch in a direction substantially perpendicular to, andaway from wafer 502. An annular ring portion 704 extends radiallyoutward approximately seven tenths (0.7) of an inch from cylindricalsheath portion 702. The annular ring portion 704 terminates in an outerlip 706 which is used to support wafer 502 during loading and unloading.A cylindrical skirt 708 extends downward approximately one eighth(0.125) of an inch from the middle of annular ring portion 704. Thiscylindrical skirt 708 fits within a cylindrical slot formed by channelpiece 622 b of the radiation shield. As shown in FIG. 6A, thecylindrical skirt helps prevent light from entering cavity 524 whileallowing pieces 622 b, 622 c, 622 d, and 622 e of the radiation shieldand wafer 502 to rotate independently of center piece 622 a, centersupport 619, and optical sensor 526.

Referring to FIGS. 6A and 6B, channel piece 622 b of the radiationshield sits approximately sixty thousandths (0.060) of an inch below thecenter piece 622 a during processing. This provides a channel 617 intocavity 524 between center piece 622 a and channel piece 622 b. Thechannel 617 is shaped with one or more ninety degree (90°) turns to helpprevent external light from entering cavity 524. Each additional turnhelps prevent light from reflecting into cavity 524. In the firstembodiment, the channel has six (6) ninety degree (90°) turns. Thechannel 617 provides an exit path for any purge gases introduced intocavity 524. Even when purge gases are not used, the channel 617substantially prevents deposits from forming inside cavity 524. Thereactive gases are depleted within the narrow channel well beforeentering cavity 524. The channel 617 also allows pieces 622 b, 622 c,622 d, and 622 e of the radiation shield and wafer 502 to rotateindependently of center piece 66 a, center support 619, and opticalsensor 526. Channel piece 622 b (and in turn pieces 622 c, 622 d, and622 e and wafer 502) are supported by a quartz rotating support 620. Therotating support 620 comprises three narrow legs as shown in FIG. 6Binstead of a single solid cylinder in order to reduce thermal insulationwhich might otherwise interfere with the heating of wafer 502. Therotating support 620 rotates during processing at approximately twenty(20) rotations per minute. This in turn rotates wafer 502 which helpseven out any non-uniformities in heating due to uneven intensities inlamps 516. In addition, rotation helps average out localized backsidesurface roughness and temperature irregularities that might affect theflux density detected by optical sensor 526 (which remains stationaryduring processing). Rotation also enhances the uniformity of layersdeposited on the substrate by evening out localized irregularites whichmay be caused by, among other things, uneven gas flow distributionacross the substrate surface.

FIG. 7C is a side cross sectional view, and FIG. 7D is a bottom planview, of channel piece 622 b of the radiation shield. Referring to FIGS.7C and 7D, a cylindrical slot 710 is formed by channel piece 622 b whichis shaped to receive cylindrical skirt 708 of center piece 622 a.Channel piece 622 b also comprises an annular ring portion 712 extendingradially outward approximately four tenths (0.4) of an inch fromcylindrical slot 710 and terminating in a lip 714. The lip 714 is usedto hold cylindrical piece 622 c of the radiation shield in place asshown in FIG. 6A. Cylindrical piece 622 c is a cylinder of siliconcarbide approximately fifty thousandths (0.050) of an inch thick andapproximately three eighths (0.375) of an inch high. Cylindrical piece622 c is used to space center piece 622 a and channel piece 622 b of theradiation shield farther from wafer 502 than the rest of the radiationshield to avoid uneven heating in the center of wafer 502. Since centerpiece 622 a overlaps channel piece 622 b, the radiation shield providesbetter thermal insulation in its central region. To compensate for anyeffect this may have on the heating of wafer 502, this central region isspaced farther from the wafer than the rest of the radiation shield toreduce its impact on wafer heating. The distance from the wafer may beadjusted as necessary to allow for substantially uniform wafer heating.Alternatively, the center of the radiation shield may be constructedfrom thinner pieces of silicon carbide to reduce the effect of theoverlapping pieces and provide a more uniform thermal insulation beneaththe wafer.

On top of cylindrical piece 622 c of the radiation shield is placed anannular ring piece 622 d which extends radially outward substantiallyparallel to, and approximately three eighths (0.125) of an inch below,wafer 502. The annular ring piece 622 d has a width of approximatelyfifty thousandths (0.050) of an inch. The annular ring piece 622 d alsohas an inner downward lip to hold cylindrical piece 622 c in place andan outer upward lip which holds the outer piece 622 e of the radiationshield in place.

The outer piece 622 e of the radiation shield supports the outer edge ofwafer 502. FIG. 7E is a side cross-sectional view, and FIG. 7F is abottom plan view, of outer piece 622 e. As shown in FIGS. 7E and 7F,outer piece 622 e has a recessed shelf region 716 for receiving andsupporting the edge of wafer 502. Only a small portion (approximately afew millimeters) of the wafer edge rests on this shelf region 716. Inthe first embodiment, this is the only portion of the radiation shieldthat makes contact with wafer 502 during processing. Extending downwardfrom shelf region 716 is a cylindrical support portion 718 which restson the annular ring piece 622 d of the radiation shield as shown in FIG.6A. As shown in FIGS. 7E and 7F, an annular ring portion 720 of theouter piece 622 e is elevated relative to the step portion byapproximately sixty thousandths (0.060) of an inch. The annular ringportion 720 extends radially outward from the wafer edge effectivelyproviding a thermal extension to wafer 502. This annular ring portion720 provides extra thermal insulation to retain heat at the outer edgeof wafer 502 where heat losses are the greatest due to the proximity ofthe chamber walls. Referring to FIGS. 6A and 6B, a resistive siliconcarbide coated graphite peripheral heater 528 may be placed below outerpiece 622 e to provide additional heat to compensate for heat losses atthe edge of wafer 502. The intensity of radiant energy heating sourcespositioned above the periphery of the wafer may also be increased tocompensate for edge losses. However, a resistive heater providesadvantages over many radiant energy heating sources that use linearfilaments, since the resistive heater provides a uniform circumferentialband of heat at a peripheral region of the wafer edge where heat lossesare the greatest. In contrast many radiant energy heating sources uselinear filaments which-provide heat in linear segments and as a resultare ineffective or inefficient at providing a uniform circumferentialband of heat.

FIG. 8 is a side cross sectional view of an alternative embodiment of arapid thermal processing chamber according to the present invention.FIG. 8 illustrates the same design as FIG. 6A except that a plurality ofoptical sensors 826, 850 and 860 are deployed at various radii acrossthe wafer surface. The components in FIG. 8 that correspond tocomponents in FIG. 6A are referenced using the same number except thatan eight is used in the hundreds decimal place for clarity. Additionaloptical sensors 850 and 860 are preferably optical fibers in thealternative embodiment. The optical fibers are relatively stiff andsupport themselves within the cavity. The end of each optical fiber isbeveled to direct light from the wafer surface into the correspondingoptical sensor from which the temperature of the wafer at a given radiusmay be determined.

Only a single optical sensor is used to measure the temperature of thewafer at each radius since the wafer rotates and the optical sensorthereby collects light that is an average of the light emitted by thewafer along the corresponding radius. The intensity sampled at eachradius (or the temperature calculated therefrom) may then be compared todetect temperature differences across the wafer surface. In particular,the intensity of light sampled by optical sensor 850 at the wafer edgemay be compared to the intensity of light sampled by optical sensor 826at the center of the wafer. The heat provided by ring heater 828 maythen be adjusted to compensate for any difference in temperature betweenthe center and edge of wafer 802. In addition (or alternatively), theintensity of lamps or other radiant energy heating sources positionedabove the periphery of the wafer may be controlled to adjust fortemperature differences. See, e.g., U.S. Pat. No. 5,268,989 to Moslehiet al. (“Moslehi”) which is incorporated herein by reference. A lampsystem similar to that described in Moslehi may be used to compensatefor temperature differences detected by optical sensors deployed in ashielded region according to the present invention. Two or more opticalsensors may be positioned in the shielded region to sample theintensity-of light at various positions along the wafer surface. Thenumber of optical sensors may vary depending upon the number ofindividually controlled lamp modules or other heaters that are used.

FIG. 9 illustrates an alternative passive method for compensating foredge heat losses. FIG. 9 illustrates the same design as FIG. 6A exceptthat the radiation shield provides more thermal insulation near theperiphery of wafer 902 and no active peripheral heater is used. Thecomponents in FIG. 9 that correspond to components in FIG. 6A arereferenced using the same number except that a nine is used in thehundreds decimal place for clarity. The extra thermal insulation isprovided by adding annular rings 950 to the outer piece 922 e of theradiation shield as shown in FIG. 9. The amount of extra thermalinsulation at the periphery of the wafer may be adjusted by addingannular rings as necessary to compensate for average edge losses in aparticular reactor design.

In each of the above embodiments, the radiation shield (522, 822 and922) and the wafer (502, 802 and 902) substantially prevent light frombeing transmitted into the cavity (524, 824 and 924) at the wavelengthsused for optical pyrometry. The radiation in the cavity is predominantlyemitted by the wafer and radiation shield. Therefore, the cavityapproximates an ideal black-body cavity radiator and the temperature ofthe cavity may be determined using the equations listed in Table 1.

TABLE 1 (1)$L = \frac{K\quad ɛ^{1}C_{1}}{\lambda^{5}\left\lbrack {{\exp \left( \frac{C_{2}}{\lambda T} \right)} - 1} \right\rbrack}$

(2) $ɛ^{1} = \frac{ɛ}{{ɛ\left( {1 - {\alpha/S}} \right)} + {\alpha/S}}$

The first equation in Table 1 is Planck's equation which defines therelationship between the flux density of light emitted from an objectand the temperature of that object. For the form of Planck's equationlisted in Table 1, L is the radiance of light at a given wavelength inw/m; λ is the wavelength of light; K is a physical constant relating tothe shape, light transmitting properties and dimensions of the opticalsensor; T is the temperature of an object in degrees Kelvin; ε¹ is theeffective (or apparent) emissivity of the object; C₁ is the firstradiation constant, 0.59544×10⁻¹⁶ w·m²; and C₂ is the second radiationconstant, 1.438786×10⁻² mK. The second equation in Table 1 is anequation for determining the effective emissivity, ε¹, of a cavityradiator where ε is the intrinsic emissivity at a given temperature ofthe material comprising the cavity walls; S is the surface area of thecavity; and α is the surface area of an aperture into the cavity(through which the flux density of light is measured). In an idealcavity radiator α/S approaches zero (ø), so the effective emissivity ofthe cavity is one (1) regardless of the shape of, or materials used for,the cavity walls. In addition, an ideal cavity radiator is assumed tohave walls of equal temperature.

In the first embodiment, the temperature of wafer 502 is calculatedbased upon the flux density of light sampled by optical sensor 526.Optical sensor 526 has a cone of vision with an angle of approximatelyfifty degrees (50°), so optical sensor 526 detects light over an area ofwafer 502 as indicated at 625 in FIG. 6A. Since wafer 502 is rotating,localized surface roughness and temperature irregularities may beaveraged out to some degree particularly if the cone of vision foroptical sensor 526 is offset slightly from the center of wafer 502.Planck's equation as shown in Table 1 (or an approximation based onPlanck's equation) may then be used to calculate the temperature ofwafer 502 from the measured flux density of light. Conventional opticalpyrometry techniques may be used for this purpose. See, e.g., U.S. Pat.No. 4,845,657 to Dils et al. which is incorporated herein by reference.

During thermal equilibrium when the walls of cavity 524 (including wafer502) are all at substantially the same temperature, the second equationin Table 1 may be used to calculate the effective emissivity which maythen be used by the optical pyrometer to determine temperature. It isdesirable to configure the cavity to have an effective emissivitygreater than the intrinsic emissivity of the wafer for a givenwavelength and processing temperature. For instance, with a siliconwafer having intrinsic emissivity of approximately seven tenths (0.7) ata one (1.0) micrometer wavelength for most processing temperatures, itis desirable to configure the cavity to raise the effective emissivityof the cavity above nine tenths (0.9). As the effective emissivity isfurther increased toward unity (i.e. from 0.95 to 0.96 to 0.97 to 0.98and above) further advantages are realized as the temperaturemeasurement becomes less dependent on the intrinsic emissivity of thewafer. To achieve these advantages, it is desirable to decrease theratio of the aperture area to the surface area of the cavity (α/s) toone tenth (0.1) or less. In the first embodiment, the aperture into thecavity 524 for measuring the flux density of light is a cross section ofthe shaft through which optical sensor 526 is introduced into cavity 524and the area of that aperture, α, is on the order of 0.0039π squareinches. The internal surface area of the cavity as a whole, S, on theother hand is greater than twice the surface area of wafer 502, whichfor a two hundred millimeter (200 mm) wafer is approximately 32π squareinches. Therefore, the ratio of the aperture area to the cavity surfacearea, α/S, is on the order of 0.0039/32 (approximately 0.00012) and theeffective emissivity calculated from the second equation in Table 1 isclose to unity and is substantially independent of the intrinsicemissivity, ε, of the cavity walls. Therefore, the calculation of wafertemperature (near thermal equilibrium) in the first embodiment issubstantially insensitive to the intrinsic emissivity of wafer 502.Importantly, this allowsa wide range of wafer types, materials, dopantlevels and layers to be processed without substantially affecting theaccuracy of the temperature measurement.

However, the cavity 524 will deviate from ideal cavity radiatorconditions particularly during temperature ramp up and ramp down. Duringtemperature ramp up, the wafer 502 (which is exposed directly to lampradiation) will heat up more quickly than radiation shield 522 (most ofwhich is not directly exposed to lamp radiation). For instance,initially the flux density of light detected at the surface of wafer 502will correspond to the intrinsic emissivity of wafer 502 (rather than ahigher effective emissivity of the cavity), since radiation shield 522may not yet be contributing to the emitted light at a levelcorresponding to the temperature of the wafer. However, even outside ofthermal equilibrium conditions, the first embodiment is relatively freefrom the effects of complex and dynamic changes in the emissivity ofwafer 502. This is accomplished in the first embodiment by choosing awavelength for optical pyrometry at which the emissivity of wafer 502 isrelatively stable over a range of temperatures. A wavelength in therange of eight tenths (0.8) to one and two tenths (1.2) micrometers isused in the first embodiment, with a wavelength of one (1.0) micrometerbeing particularly advantageous, since the emissivity of a pure siliconwafer (and certain doped and layered silicon wafers) is relativelystable at these wavelengths. In addition, the transmission of puresilicon (and most other silicon and non-silicon based semiconductormaterials) at these wavelengths is relatively low. In addition, dynamicchanges in the emissivity of wafer 502 due to deposition of layers onthe backside of wafer 502 is eliminated in the first embodiment, sinceradiation shield 522 prevents deposits from forming on the backside ofwafer 502.

Thus, it is believed that even outside of thermal equilibrium thetemperature of wafer 502 may be accurately determined without complexdynamic dependencies upon the intrinsic emissivity of wafer 502. To theextent that the temperature of the radiation shield 522 lags thetemperature of wafer 502, and therefore does not emit sufficient lightto raise the effective emissivity close to unity, it is believed that alinear (or near linear) adjustment may be made to the temperaturecalculation based upon the emissivity and other thermal properties ofthe silicon carbide radiation shield (which may be determined inadvance). In addition, calibration light source 540 may be used todetermine the intrinsic emissivity of the wafer before processing whichmay also be used to adjust the temperature calculation. In the firstembodiment, the lag in heating of the radiation shield 522 relative towafer 502 due to indirect exposure to lamps 516 is mitigated by heatprovided by peripheral heater 528 below the radiation shield. Inaddition (or in alternative), other embodiments may use radiant energyheating sources or other heaters positioned below the shield tocompensate for uneven heating. As described above, the peripheral heateralso helps compensate for heat losses at the wafer edges. The need foradjustments to the temperature calculation will depend upon a variety offactors related to the particular reactor design including relativeexposure of the wafer and radiation shield to heaters, as well as theshape, thickness and material(s) used for the radiation shield. However,it is anticipated that any required adjustment may be determinedexperimentally in advance rather than requiring complex dynamicadjustments based on direct measurements of emissivity and otherconditions as are required in some conventional systems. A thermocoupleor similar device may be used to calibrate the optical pyrometer anddetermine any necessary adjustment prior to processing.

Although some adjustment may be necessary for temperature measurementduring periods of non-equilibrium, accurate temperature measurement maybe achieved in the first embodiment during processing steps where asteady and uniform temperature is desired. During these criticalprocessing steps important advantages are realized, since substantialthermal equilibrium is achieved and temperature measurement becomessubstantially insensitive to emissivity. Moreover, during periods oframp up and ramp down multi-point pyrometry (as described above withreference to FIG. 8) may be used to identify and compensate forundesired temperature nonuniformities even if absolute temperature isnot calculated. Rather than calculating absolute temperature, thedifference in the flux density of light detected by the three (or more)optical sensors 826, 850 and 860 may be used to detect temperaturedifferences. Peripheral heater 828, lamp modules, or other heaters maythen be adjusted as necessary to provide more uniform heating.

In addition to simplifying the measurement of wafer temperature,radiation shield 522 also prevents the undesired deposition of materialson the backside of wafer 502 during processing. Reactive gases (or othersubstances) introduced into processing chamber 504 through inlet 508 aresubstantially prevented from entering cavity 524. It should be notedthat RTP chemical vapor deposition may occur at a variety of pressuresincluding vacuum conditions at less than twenty (20) Torr (and oftenless than one (1) Torr). In addition, any gas in cavity 524 may expandwhen heated. Thus, a channel between processing chamber 504 and cavity524 (such as channel 617 formed between center piece 622 a and channelpiece 622 b) is desired to allow pressure equalization duringprocessing. However, channel 617 should be sufficiently narrow toprevent deposition from occurring within cavity 524 and should be shapedto substantially prevent reflected light from entering cavity 524. Inthe first embodiment, channel 617 includes a plurality of turns and hasa width of approximately sixty thousandths (0.060) of an inch. Thisnarrow channel inhibits the incidental reflection of light into thecavity and depletes reactive gases well before they reach the cavity.

A purge gas, such as hydrogen, may also be introduced into cavity 524through sheath 615 surrounding optical sensor 526. The purge gas exitsthe cavity through channel 617 and may help prevent deposits within thechannel. However, due to the narrow width of channel 617 in the firstembodiment, it is believed that purge gases are unnecessary for thispurpose. In fact, the use of purge gas is limited in the firstembodiment to avoid potential cooling effects within cavity 524. Only asmall amount (on the order of 100 SCC/min.) is used to avoid deposits onoptical sensor 526 from silicon or other material vaporized from wafer502 during processing at low pressures and high temperatures (generallygreater than 800° C.). Typically, purge gas is not used in the firstembodiment to prevent deposition from reactive gases in processingchamber 504. Rather, radiation shield 522 and channel 617 prevent suchdeposits. In fact, it is believed that purge gases are not necessary forsome processes (generally at high pressures and temperatures less than800° C.).

When purge gases are used, they may be introduced through a centralsupport and drive mechanism which also provides the rotational drive forrotating support 620 and the lift mechanism for center piece 622 a. Thiscentral support and drive mechanism will now be described with referenceto FIG. 10. FIG. 10 is a side cross sectional view of a central supportand drive mechanism, generally indicated at 530, according to the firstembodiment of the present invention. The mechanism 530 is positionedbelow processing chamber 504. A stainless steel cylindrical housing cup1002 is bolted to the underside of processing chamber 504. A hole 514(not shown in FIG. 10) is formed in the bottom wall of the processingchamber over the housing cup 1002. The rotating support 620, centersupport 619, sheath 615, and optical sensor 526 pass through this holeand into processing chamber 504 as shown in FIG. 5. A seal (not shown)is placed between housing cup 1002 and the bottom wall of processingchamber 504 to provide a vacuum seal.

A stainless steel rotating pedestal 1004 sits within the housing cup1002. The rotating pedestal 1004 is held in place by two cylindricalbearings 1006 and 1008 which allow the rotating pedestal 1004 to rotaterelative to the housing cup 1002. A stainless steel cylindrical spacer1010 is used to facilitate the placement of bearings 1006 and 1008. Anupper portion of the rotating pedestal 1004 sits above bearings 1006.This upper portion of the rotating pedestal 1004 holds a plurality ofmagnets 1012 which are disposed in a ring radially about the upperportion of the rotating pedestal 1004. Attached to the top surface ofthe rotating pedestal 1004 is a threaded cylindrical base support 1014which receives the base of the quartz rotating support 620. A thinresilient pad (not shown) placed inside base support 1014 acts as acushion between the quartz rotating support 620 and the stainless steelbase support 1014. The base of the rotating support 620 is held in placeby a threaded stainless steel cover 1016 which screws onto cylindricalbase support 1014. A rubber o-ring 1018 or other resilient cushion isplaced between cover 1016 and the base of the rotating support 620 tohold the base of the rotating support in place without having to clampthe stainless steel cover directly against the quartz rotating support620. The rubber o-ring also provides flexibility to allow for expansionand contraction of components.

Around the outside of the housing cup 1002 is placed a cylindrical gear1020. A cylindrical bearing 1021 is placed between-the gear 1020 and thehousing cup 1002 to reduce friction. Outer magnets 1022 are mounted ontop of the gear 1020 about a radius that is outside of, but slightlybelow, the ring of inner magnets 1012. The outer magnets 1022 and innermagnets 1012 are magnetically coupled through housing cup 1002. A belt(not shown) drives gear 1020 causing gear 1020 and outer magnets 1022 torotate about the housing cup 1002. During processing the belt is drivenat a rate that will cause rotation at approximately twenty (20)rotations per minute. This, in turn, causes the inner assembly (whichincludes magnets 1012, rotating pedestal 1004, base support 1014, cover1016, o-ring 1018, and rotating support 620) to rotate due to themagnetic coupling between inner magnets 1012 and outer magnets 1022. Inaddition, the magnetic coupling imposes a slight downward force on theinner assembly, since the outer magnets are slightly lower than theinner magnets. This helps hold the inner assembly in place.

The center support 619, sheath 615, and optical sensor 526 pass througha shaft in the center of the inner assembly. The base of the centersupport 619 is clamped to a stainless steel elevational base support1024. O-rings 1026 are placed between the center support 619 and theelevational base support 1024 to prevent the stainless steel elevationalbase support 1024 from clamping directly against the quartz centersupport 619. The sheath 615 and optical sensor 526 are also coupled tothe elevational base support 1024 as indicated at 1024. A bellows 1028is clamped in place between elevational base support 1024 and housingcup 1002. Rubber o-rings 1030 and 1032 are used to hold bellows 1028 inplace while allowing some flexibility for expansion and contraction ofcomponents. A pneumatic or electromechanical drive (not shown) is usedto raise and lower the elevational base support 1024 in order to raiseand lower center support 619, sheath 615, and optical sensor 526. Whenelevational base support 1024 is raised, bellows 1028 contracts and whenelevational base support 1024 is lowered, bellows 1028 expands. Thiselevational mechanism is used to raise and lower center piece 622 a ofthe radiation shield in order to load and unload wafer 502 from theprocessing chamber and to prevent bellows 1028 from contracting undervacuum pressure.

A purge gas, such as hydrogen, may be introduced through inlet 1034 toprevent reactive gases from causing deposits within mechanism 530. Apurge gas may also be introduced at 1036 through the sheath 615 toprevent deposits on optical sensor 526.

A typical RTP process will now be described with reference to FIGS. 8and 10. During rapid thermal processing, the elevational base support1024 (and in turn center piece 822 a) starts out in a raised position.Initially, gear 1020 (and pieces 822 b, 822 c, 822 d, and 822 e of theradiation shield) is not rotating. A wafer 802 is placed on raisedcenter piece 822 a through a conventional load lock mechanism. Theelevational base support 1024 is then lowered, which lowers the centerpiece 822 a and places the wafer edges on outer piece 822 e of theradiation shield. Gear 1020 is then rotated at approximately twenty (20)rotations per minute which in turn causes outer piece 822 e and wafer802 to rotate at the same rate. For one common RTP process, the lamps816 and peripheral heater 828 are then ramped up to about eight hundreddegrees Celsius (800° C.) over a period of about ten (10) seconds.During this ramp up period, the power to the peripheral heater 828 (orthe lamps) may be adjusted to compensate for any difference in intensitydetected by optical sensors 826, 850 and 860. The lamps and peripheralheater are then held at a level of approximately eight hundred degreesCelsius (800° C.) for about one (1) minute to allow the temperature ofthe system to stabilize before further increases in temperature. Opticalsensors 826, 850 and 860 are used to monitor the temperature of wafer802 during this period of time. Minor adjustments may be made to lamps816 and peripheral heater 828 to maintain the wafer at this temperatureand to adjust for any temperature nonuniformities. The temperature ofwafer 802 is then gradually ramped up to a desired temperature forprocessing over a period of about one (1) minute. Typically, the waferis heated to a temperature in the range of from about eight hundreddegrees Celsius (800° C.) to about one thousand two hundred degreesCelsius (1200° C.), with a temperature of one thousand one hundreddegrees Celsius (1100° C.) being common. Optical sensors 826, 850 and860 are used to monitor the temperature of wafer 802 and adjust for anynonuniformities during this period of time. A processing step, such asdeposition, anneal or the like, may then be performed. A substance forchemical or physical vapor deposition, etching, or other processing maybe introduced into the chamber through inlet 808 or other conventionalmechanisms. Typically the temperature is held constant duringprocessing. This processing step typically lasts anywhere from tenseconds to several minutes. The temperature is then ramped down over aperiod of about thirty (30) seconds. Once again, optical sensors 826,850 and 860 are used to monitor the temperature of wafer 802 and adjustfor any nonuniformities. Normally as a wafer cools in a cold wall RTPchamber, there will be large heat losses at the wafer edges due to theproximity of the walls. In order to compensate for this effect, thetemperature of peripheral heater 828 is carefully controlled based onthe measurements of optical sensors 826, 850 and 860. Even after thepower to the lamps 816 and ring heater 828 has been shut off, the wafermay be left in place for a short period of time. During this period oftime, the thermal insulation provided by the peripheral heater 828continues to compensate for edge heat losses which allows for moreuniform cooling. At this point, the rotation of gear 1020 and wafer 802may be discontinued. The elevational base support 1024 is then raised,which raises the center piece 822 a and wafer 802. The wafer is thenremoved through a conventional load lock mechanism. The wafer maycontinue to cool at a different location while another wafer is loadedinto chamber 804 for processing. It will be readily understood that theabove description of a typical RTP process is exemplary only, and thatany variety of time periods, temperatures, and steps may be used forprocessing.

The foregoing description is presented to enable any person skilled inthe art to make and use the invention. Descriptions of specific designsare provided only as examples. Various modifications to the preferredembodiment will be readily apparent to those skilled in the art, and thegeneric principles defined herein may be applied to other embodimentsand applications without departing from the spirit and scope of theinvention. Thus, the present invention is not intended to be limited tothe embodiments shown, but is to be accorded the widest scope consistentwith the principles and features disclosed herein.

For instance, it will be readily apparent to those of ordinary skill inthe art that aspects of the present invention may be applied in anyvariety of semiconductor substrate processing systems using a wide rangeof heaters, reactive gases, pressures and temperatures. For example,while particular advantages are realized when using aspects of theinvention in a radiant energy heating source RTP system, aspects of theinvention may be applied to systems using other heaters, includingwithout limitation contact or proximity resistive heaters or reflectionor diffusion radiant energy heating systems.

Moreover, under certain circumstances a shielded region providing aneffective black-body cavity may be provided adjacent to only a portionof a semiconductor substrate rather than across the entire substratesurface. A shielded region may also be provided by a shield that doesnot contact or entirely enclose a substrate surface. For instance, ashield may be closely spaced to the substrate with gaps or channelsformed between the shielded region and the processing chamber. Althoughthe gaps or channels may not contain turns as in the first embodiment,most light from the processing chamber may be required to follow a pathwith multiple reflections before reaching an optical sensor in theshielded region. Thus, even with openings to the processing chamber, ashield may be used to substantially prevent extrinsic light frominterfering with an optical sensor.

In addition, the interior surface of the radiation shield could beprovided with a reflective finish or coated with a reflective materialin order to create a reflective cavity with an effective emissivityapproaching unity. It is believed that such a reflective cavity willprovide results superior to conventional reflective cavities due to thesmall aperture size, isolation of the cavity from chamber walls, andisolation of the cavity from external sources of light. However, areflective cavity approach is expected to be more expensive than theapproach of the first embodiment while potentially exposing the wafer tolocalized reflective hot spots and contaminants from mirrored surfaces.In addition, the reflective surfaces may require regular cleaning tomaintain a high level of reflectivity. Other coatings may also be usedon the radiation shield to improve thermal properties. For instance, incertain embodiments it may be useful to coat the interior surface of theradiation shield with a material (such as silicon or gallium arsenide)that has properties similar to the semiconductor substrate. For certainradiation shields this may reduce the risk of contaminating thesubstrate and may cause the shielded region to behave more like an idealcavity radiator.

Another modification would be to eliminate the channel which allowsrotation and pressure equalization. The wafer could be stationary or theentire support mechanism could rotate. In addition, pressureequalization could be accomplished by gas provided and removed through acenter support rather than through a separate channel. In addition, theradiation shield may comprise a variety of materials that aresubstantially nontransmissive to light at a desired wavelength, and theradiation shield may be assembled from any variety of pieces or may beproduced from a single piece of material. Further, for some processes,the reactor chamber walls may form part of the shield for the shieldedregion. A heat source, such as a resistively heated conductive block,may also form part of the shield. For instance, a semiconductorsubstrate may be placed upon a heated block which has a small recessedregion forming a cavity below a small portion of the substrate. Anoptical sensor could be inserted into the cavity through a hole in theheated block.

In addition, enclosing the backside of a substrate according to aspectsof the present invention may be used to prevent backside deposition evenif cavity radiation is not being used to calculate temperature. In suchcases, the cavity shield may be constructed using materials having awider range of light transmissive and thermal properties. For instance,a clear quartz shield could be used in conjunction with lamps on eitherside of a wafer to allow heating from both sides of the wafer whilepreventing deposition on one side.

While this invention has been described and illustrated with referenceto particular embodiments, it will be readily apparent to those skilledin the art that the scope of the present invention is not limited to thedisclosed embodiments but, on the contrary, is intended to covernumerous other modifications and equivalent arrangements which areincluded within the spirit and scope of the following claims.

What is claimed is:
 1. A method for thermal processing of asemiconductor substrate in a processing chamber, the method comprisingthe steps of: heating the substrate; sampling light emitted from a firstsurface of the substrate within a given range of wavelengths; shieldingthe first surface of the substrate from substantially all extrinsiclight from the processing chamber within the given range of wavelengthswith a shield forming a cavity adjacent to the first surface of thesubstrate; heating the shield with thermal radiation emitted by thefirst surface of the substrate wherein the thermal radiation is absorbedby the shield; heating the cavity with thermal radiation emitted by theshield; substantially maintaining the temperature uniformity of thesubstrate with thermal insulation provided by the shield and the cavity;and controlling the heating to achieve a desired processing temperatureas a function of the sampled light emitted from the first surface of thesubstrate.
 2. The method of claim 1, wherein the step of heating thesubstrate to the desired processing temperature further comprisesrapidly heating the substrate using a radiant energy heating source. 3.The method of claim 2, wherein the radiant energy heating sourceradiates substantial light into the processing chamber within the givenrange of wavelengths.
 4. The method of claim 1, wherein the step ofsampling light emitted from the first surface of the substrate furthercomprises the step of sampling light emitted from a plurality ofdifferent areas across the first surface of the substrate.
 5. The methodof claim 4, further comprising the step of controlling the heatingprocess to achieve a substantially uniform substrate temperature as afunction of the sampled light emitted from the plurality of differentareas across the first surface of the substrate.
 6. The method of claim1, further comprising the steps of: depositing a material on a secondsurface of the substrate; and shielding the first surface of thesubstrate from the material to substantially prevent deposits fromforming on the first surface of the substrate.
 7. The method of claim 1,further comprising the step of providing a calibration light source todetermine the intrinsic emissivity of the semiconductor substrate. 8.The method of claim 1 further comprising the step of heating the shieldwith a peripheral heater.
 9. The method of claim 1, further comprisingthe step of providing a purge gas to the cavity from which extrinsiclight from the processing chamber is shielded.
 10. The method of claim1, further comprising the step of configuring the cavity to have aneffective emissivity greater than the intrinsic emissivity of the waferfor a given wavelength and processing temperature.
 11. A method ofthermally processing a semiconductor substrate in a processing chamber,the method comprising the steps of: heating the substrate; shielding afirst surface of the substrate from substantially all extrinsic lightfrom the processing chamber within a given range of wavelengths with ashield forming a cavity adjacent to the first surface of the substrate;rotating one portion of the shield relative to another portion of theshield; heating the shield with thermal radiation emitted by the firstsurface of the substrate; heating the cavity with thermal radiationemitted by the shield; sampling light emitted from the first surface ofthe substrate within the given range of wavelengths; and controlling theheating to achieve a desired processing temperature as a function of thesampled light emitted from the first surface of the substrate.
 12. Themethod of claim 11, wherein the step of sampling light emitted from thefirst surface of the substrate further comprises the step of samplinglight emitted from a plurality of different areas across the firstsurface of the substrate.
 13. The method of claim 11, further comprisingthe step of controlling the heating process to achieve a substantiallyuniform substrate temperature as a function of the sampled light emittedfrom the plurality of different areas across the first surface of thesubstrate.
 14. The method of claim 11, further comprising the steps of:depositing a material on a second surface of the substrate; andshielding the first surface of the substrate from the material tosubstantially prevent deposits from forming on the first surface of thesubstrate.
 15. The method of claim 11, further comprising the step ofproviding a calibration light source to determine the intrinsicemissivity of the semiconductor substrate.
 16. The method of claim 11,further comprising the step of heating the shield with a peripheralheater.
 17. The method of claim 11, further comprising the step ofproviding a purge gas to the cavity from which extrinsic light from theprocessing chamber is shielded.
 18. The method of claim 11, furthercomprising the step of configuring the cavity to have an effectiveemissivity greater than the intrinsic emissivity of the wafer for agiven wavelength and processing temperature.
 19. A method of thermallyprocessing a semiconductor substrate in a processing chamber, the methodcomprising the steps of: heating the substrate in a first temperatureramp-up step to a temperature of about 800 degrees Celsius or less;heating the substrate in a second temperature ramp-up step to aprocessing temperature of from about 800 degrees Celsius to about 1200degrees Celsius; maintaining the temperature of the substrate in aprocessing step within the range 800 degrees Celsius to about 1200degrees Celsius; cooling the substrate in a temperature ramp-down step;monitoring the temperature of the substrate during the first and secondtemperature ramp-up steps, the processing step, and the temperatureramp-down step, with an optical sensor configured to sample lightemitted from a first surface of the substrate; shielding the firstsurface of the substrate from substantially all extrinsic light from theprocessing chamber within a given range of wavelengths with a shieldforming a cavity adjacent to the first surface of the substrate; heatingthe shield with thermal radiation emitted by the first surface of thesubstrate; heating the cavity with thermal radiation emitted by theshield; and controlling the heating to achieve a desired processingtemperature as a function of the sampled light emitted from the firstsurface of the substrate.
 20. The method of claim 19, wherein the firsttemperature ramp-up step occurs over a period of about 10 seconds orless.
 21. The method of claim 19, wherein the second temperature ramp-upstep occurs over a period of about one minute.
 22. The method of claim19, wherein the temperature ramp-down step occurs over a period of about30 seconds.
 23. The method of claim 19, further including the step ofrotating one portion of the shield relative to another portion of theshield.
 24. The method of claim 19, wherein the step of sampling lightemitted from the first surface of the substrate further comprises thestep of sampling light emitted from a plurality of different areasacross the first surface of the substrate.
 25. The method of claim 19,further comprising the step of controlling the heating process toachieve a substantially uniform substrate temperature as a function ofthe sampled light emitted from the plurality of different areas acrossthe first surface of the substrate.
 26. The method of claim 19, furthercomprising the steps of: depositing a material on a second surface ofthe substrate; and shielding the first surface of the substrate from thematerial to substantially prevent deposits from forming on the firstsurface of the substrate.
 27. The method of claim 19, further comprisingthe step of providing a calibration light source to determine theintrinsic emissivity of the semiconductor substrate.
 28. The method ofclaim 19, further comprising the step of heating the shield with aperipheral heater.
 29. The method of claim 19, further comprising thestep of providing a purge gas to the cavity from which extrinsic lightfrom the processing chamber is shielded.
 30. The method of claim 19,further comprising the step of configuring the cavity to have aneffective emissivity greater than the intrinsic emissivity of the waferfor a given wavelength and processing temperature.